Tumors induced by transplants, viruses, chemicals and radiation are studied in mice. Prevention of these tumors by various vaccines is subsequently tested. Mouse strains of low spontaneous cancer incidence, resembling the human experience are used in most studies. Evidence of xenotropic type C virus has been found in all murine tumors. Vaccines prepared from tumors and cultured cells have produced promising protection from challenge. New methods of producing killed vaccines are compared to the standard formalin inactivation. Experimental xenografting to athymic nude mice (which are hereditarily immunodeficient) continues to prove an excellent measure of cell malignancy and transformation. BIBLIOGRAPHIC REFERENCES: Arnstein, P., Riggs, J.L., Oshiro, L.S., Huebner R.J., and Lennette, E.H.: Induction of lymphoma and associated xenotropic type C virus in C57L mice by whole-body irradiation. J. Natl. Cancer Inst. 57: 1085-1090, 1976. Neubauer, R.H., Rabin, H., Arnstein P., Woodside, N., Valerio, M.G., Wallen, W.C., Hopkins, R.F., III, and Loeb, W.F.: Characterization of a spontaneous undifferentiated carcinoma from an African green monkey (Cercopithecus aethiops). In Vitro 12: 533-539, 1976.